Cyanobacteria perform two fundamental processes which allow life to exist on Earth: the conversion of light energy into chemical energy with the concomitant production of oxygen and the subsequent dark fixation of CO2 into biomass. The long-term objectives of this research program are to develop a detailed understanding of the responses of these important organisms to environmental stresses, including low- temperature-induced stresses, and to understand the role of multiple Group 2 and 3 sigma factors in the required gene regulatory responses. Most of the proposed studies will be performed using Synechococcus sp. PCC 7002, a physiologically well characterized, rapidly growing, transformable, unicellular, marine cyanobacterium. In the next funding period we will complete the overproduction of all sigma factors for antiserum production, and we will continue our biochemical and physiological analyses of mutants lacking specific sigma factors. The antisera will be used to characterize the expression patterns of sigma factor proteins in cells grown in normal batch cultures or under light- dark cycles and in cultures subjected to a variety of environmental stresses. A global transcription mapping method will be used to identify genes whose transcription is regulated by specific sigma factors. These studies should greatly enhance our understanding of transcription regulation in cyanobacteria, which are unusual among eubacteria in possessing multiple Group 2 sigma factors. During the present funding period we showed that cyanobacteria experience N-limitation when grown at low temperatures and that membrane lipid desaturation extends the growth range to lower temperatures by allowing nitrate assimilation to continue at temperatures below 22 degrees C. The rate-limiting step in nitrate assimilation at low temperature will be determined, and we will determine whether other transport functions are functionally impaired at low temperature or when lipid desaturation is reduced. The membrane fluidity of Synechococcus sp. PCC 7942 will be manipulated to test whether an increase in lipid desaturation can improve transport functions at low temperature. Other adaptive changes that allow cyanobacteria to grow at low temperature will be sought. These studies should help to define parameters that limit bacterial growth at low temperature. In the final aim we will complete the characterization of the rapid peroxidative death phenomenon that we discovered in the current funding period. These last studies could explain bloom disappearance in natural populations and could suggest new approaches for the control of problematic or toxic cyanobacteria in water supplies.